INFLUENCE OF HEAT TREATING AND SURFACE TREATMENTS ON THE FATIGUE STRENGTH OF A HIGH PERFORMANCE SINTERED STEEL

摘要

The fatigue properties of a high performance FLNC-4408 sinter hardenable steel were studied. Two premixed powders containing 0.75% and 0.85% graphite were pressed at 7.05 g/cm~3 and sintered at 1135℃ (2075℉) for 30 minutes in an industrial belt furnace under a 95% N_2 - 5% H_2 atmosphere. Some specimens were sintered under the same conditions but were subjected to a rapid cooling. A part of the specimens sintered without rapid cooling was tested in the as sintered condition. A second part was subjected to a nitrocarburization treatment and a third part to a carbonitriding treatment (using unaided free air cooling instead of oil quenching). Mechanical tests were conducted on the different samples to determine tensile, flexural and impact properties. Fatigue endurance limit was studied using rotating beam fatigue tests in conformance with the MPIF standard 56. The tests revealed a positive effect of the rapid cooling and, to a certain degree, of the modified carbonitriding on the fatigue strength in comparison with conventionally sintered parts.